1. Field of the Invention
The present invention relates to a wireless access system, and more particularly to a variety of frame structures and a method for transmitting information using the frame structures.
2. Discussion of the Related Art
Hereinafter, a general frame structure for use in a wireless access system will be described in detail.
FIG. 1 is a structural view illustrating a frame structure for use in a broadband wireless access system (e.g., IEEE 802.16).
Referring to FIG. 1, a horizontal axis of a frame represents an Orthogonal Frequency Division Multiplexing Access (OFDMA) symbol as a time unit and a vertical axis of the frame indicates a logical number of a sub-channel as a frequency unit. In FIG. 1, one frame is divided into data sequence channels during a predetermined time period by physical characteristics. That is, one frame includes one downlink (DL) sub-frame and one uplink (UL) sub-frame. A Transmit Transition Gap (TTG) is interposed between a DL subframe and a UL subframe, and a Receive Transition Gap (RTG) is interposed between frames.
In this case, the DL subframe may include a preamble, a Frame Control Header (FCH), a DL-MAP, a UL-MAP, and one or more DL data bursts. The UL subframe may include uplink control channels, such as a HARQ ACK channel, a fast feedback channel, and a ranging sub-channel, and one or more UL data bursts.
In FIG. 1, the preamble is specific sequence data located at a first symbol of each frame and is used to perform synchronization of a mobile station with a base station or to estimate a channel state of the base station. The FCH is used to provide channel allocation information and channel code information related to the DL-MAP. The DL-MAP/UL-MAP is a media access control (MAC) message used for informing a mobile station of channel resource allocation in downlink/uplink (DL/UL). In addition, the data burst represents the unit of data which is transmitted from a base station to a mobile station or from a mobile station to a base station.
A downlink channel descriptor (DCD) which may be used in FIG. 1 is one of MAC messages indicating the physical characteristics of a DL channel and an uplink channel descriptor (UCD) is one of MAC messages indicating the physical characteristics of a UL channel.
In downlink, referring to FIG. 1, the mobile station detects the preamble transmitted from the base station and performs synchronization with the base station. Thereafter, the DL-MAP may be decoded using information acquired from the FCH. The base station may transmit scheduling information for DL or UL resource allocation to the mobile station in each frame (e.g., 5 ms) using the DL-MAP or UL-MAP message.
With the increasing development of wireless access systems, there may arise differences between a general frame structure and an advanced frame structure. In this case, it is necessary for the newly advanced system to be compatible with a general system (e.g. legacy system), resulting in the implementation of natural communication. That is, if the newly advanced broadband wireless access system (e.g., a multi-carrier system) supports a conventional wireless access system (e.g., a single-carrier system or legacy system), a new system needs to transmit information associated with the conventional system to mobile stations, such that the mobile stations can be normally operated.
For example, assuming that a sub-MAP (i.e., scheduling information for resource allocation) for use in a new system is transmitted at every DL sub-MAP, a frame structure of a legacy system may be different from that of the new system. In other words, assuming that a mobile station supporting a new system does not recognize a frame structure of the legacy system, the mobile station may sometimes not decode a sub-MAP transmitted from the new system.
Therefore, in order to allow a mobile station applied to the new system to properly utilize radio resources, it is preferable that the mobile station recognize information (e.g., bandwidth, center frequency, DL ratio with 16e, UL ratio with 16e, etc.) of a frame structure of the legacy system.